For a full head of dreads from human hair, I have found that you need 6 feet of weft at least. If you could get 10 feet wide of weft, that would be even better.
If she is giving that hair away, take as much as you can, though, seriously! You can sell it or use it later on, and typically human hair is really expensive so if I were you I’d take as much as her generosity would allow. I hope that doesn’t sound too selfish.
I hope this is helpful to you!